Weight factor - control via keyboard
To change the Weight value press W (being in Weight Paint mode) and drag the cursor inward/outwards the small circle showing the weight value. After pressing W you may also type the value using numerical keyboard (e.g. you press W, then 1) and the weight value changes to 1).


Remember that you may also use shortcuts to change the Radius (press F) and Strenght values (press Shift+F). Use these to work faster.